Multi-junction solar cells based on III-V compound semiconductor p-n homojunctions have been proposed for the high-efficiency generation of electricity for space photovoltaic applications, and also for terrestrial high-concentration photovoltaic applications. A homojunction is formed by doping a layer of a single type of semiconductor material with n-type and p-type doped portions; whereas a heterojunction is formed from contacting layers of two different types of semiconductor materials, with the two types of semiconductor materials being oppositely doped.
For space photovoltaic applications, an increased solar cell efficiency is advantageous for increasing the available electrical power or alternately reducing satellite mass and launch cost. An increased solar cell efficiency also reduces the size required for a space photovoltaic array, thereby improving reliability by lessening he possibility for damage resulting from deployment or space debris.
Since the early 1960's, efforts have been made to improve the energy conversion efficiency of solar cells. These efforts lead to the development of multi-junction solar cells in the 1980's, and the development in 1994 of a two-junction InGaP/GaAs solar cell based on III-V compound semiconductors with an energy conversion efficiency of 29.5% for AM1.5 solar illumination at an angle of 45.degree. above the horizon (see K. A. Bertness et al., "29.5% Efficient GalnP/GaAs Tandem Solar Cells," in Applied Physics Letters, vol. 65, pp. 989-991, 1994). In 1996, a three-junction InGaP/GaAs/Ge solar cell was disclosed with an AMO (space solar spectrum) energy conversion efficiency of 25.7% (see P. K. Chiang et al., "Experimental Results of GaInP.sub.2 /GaAs/Ge Triple Junction Cell Development for Space Power Systems," in Proceedings of the 25th IEEE Photovoltaic Specialists Conference, pp. 183-186,1996).
These previous multi-junction solar cells have been based on the use of semiconductor p-n homojunctions, with each homojunction being formed from a single semiconductor material (i.e. GaInP.sub.2, GaAs or Ge) by selectively doping different portions or layers of that material p-type and n-type. For example, the above three-junction GaInP.sub.2 /GaAs/Ge solar cell comprises three p-n homojunctions (i.e. one p-n homojunction formed from each of three different semiconductor materials), with the three p-n homojunctions being series connected by intervening n-p tunnel junctions. The resulting structure is a monolithic, lattice-matched solar cell having three light-absorbing homojunctions with bandgap energies of 1.85 electron volts (eV) for the GalnP.sub.2 homojunction, 1.42 eV for the GaAs homojunction, and 0.67 eV for the Ge homojunction.
The energy conversion efficiency of this 3-junction solar cell, although large, is limited by a relatively large 0.75 eV difference in the bandgap energy of the GaAs and Ge materials which results in a significant super-bandgap energy loss to the Ge homojunction in the form of heat. Additionally, the energy conversion efficiency of the three-junction solar cell is limited by a relatively low bandgap energy of the GaInP.sub.2 homojunction which limits the number of solar photons that can reach the underlying GaAs homojunction, thereby limiting the electrical current that can be produced by the GaAs homojunction. Since each layer within the solar cell is connected in series, the electrical current limitation of the GaAs homojunction limits the overall solar cell electrical current that can be produced in response to solar illumination, thereby limiting an overall energy conversion efficiency of the device.
The need for solar cells with an even higher energy conversion efficiency has prompted the suggestion that yet another p-n homojunction should be added to the above two-junction and three-junction solar cells, with this additional p-n homojunction having a bandgap energy of about 1 eV, but otherwise with no specific identification of what type of semiconductor material could be used to provide the 1-eV homojunction (see S. R. Kurtz et al, "Projected Performance of Three- and Four-Junction Devices Using GaAs and GaInP," Proceedings of the of the 26th Photovoltaic Specialists Conference, Anaheim, Calif., Sep. 30-Oct. 3, 1997, pp. 875-878).
An advantage of the present invention is that an InGaAsN/GaAs semiconductor p-n heterojunction comprising a layer of indium gallium arsenide nitride (InGaAsN) with n-type doping epitaxially grown in contact with a layer of gallium arsenide (GaAs) with p-type doping is disclosed herein that can be used to form an efficient 0.95-1.2 eV bandgap photodetector for use in a multifunction solar cell.
Another advantage of the present invention is that the use of an InGaAsN/GaAs semiconductor p-n heterojunction having contacting layers of and p-type GaAs overcomes the limitation of a very low or negligible electron diffusion that occurs in homojunctions or heterojunctions formed in part from p-type InGaAsN.
A further advantage is that the InGaAsN/GaAs p-n heterojunction of the present invention, based on layers of n-type InGaAsN and p-type GaAs, provides a substantial increase in open-circuit voltage and short-circuit current as compared to homojunctions and heterojunctions formed in part from p-type InGaAsN.
These and other advantages of the method of the present invention will become evident to those skilled in the art.